1. Field of the Invention
The present invention relates to photographic printers. In particular, the present invention is an improved photosensor circuit for converting analog photosignals to digital values for use by a digital processor.
2. Description of the Prior Art
Photographic printers produce photographic prints from photographic film originals (generally negatives) by passing high intensity light through the film to expose the photographic emulsion layers of a photosensitive medium. The emulsion layers are subsequently processed to produce a print of the scene contained in the original.
In order to increase efficiency and minimize time required to fill customer orders, high speed printers have been developed in which many exposures are made in edge-to-edge relationship on a single roll of print paper. After the exposures are made, the roll is removed from the printer, is photoprocessed to produce prints, and is cut into individual prints. The prints are then sorted by customer order and ultimately packaged and sent to the customer.
A critical portion of a photographic printer is the exposure time control, which controls the duration of the exposure of the photosensitive medium. The exposure time control may utilize inputs from several different sources in order to determine the proper duration of the exposure. For example, most automatic printers use large area transmission density (LATD) sensors to sample the light transmitted by the negative either prior to or during the exposure. In addition, many automatic printers include an automatic density correction (ADC) or color scanning station which scans the negative prior to printing and corrects the exposure time in the event of an abnormality in illumination of the negative known as "subject failure". The operator may also enter density or color correction signals from the operator control panel. Based upon some or all of these input signals, the exposure time control determines the proper exposure time for each of the color channels based upon predetermined exposure time relationships.
In many prior art printer systems, the exposure times are controlled automatically by the LATD photocell signals by means of analog integration of the LATD photosignals. The integrated signal levels required to terminate exposure were determined by density and color controls. This technique, however, has a number of drawbacks. In particular, in most photographic systems, changes in density and color on a logarithmic rather than a linear scale. Since the analog integration technique is not logarithmic, additional circuit complexity is required to convert the settings at the density and color control, which read logarithmically, to the appropriate analog integration signal levels.
Furthermore, digital control systems provide much greater flexibility and capability for various automatic exposure corrections than are possible with an analog type system. Digital systems, however, require some form of conversion of the analog photosignal to a digital signal which is then processed by a digital processor to calculate the appropriate exposure times. In general, most digital control systems for photographic printers have used conventional analog-to-digital conversion circuits.
The photographic printer shown in U.S. Pat. No. 4,125,326, which is assigned to the same assignee as the present application, shows (in FIG. 17) another approach to exposure control on a digital basis. In this system, the sensor photosignal from each LATD sensor is supplied to a voltage controlled oscillator and controls the rate at which the oscillator produces output pulses. The output of the oscillator drives a counter which counts up until the count in the counter corresponds to a digital value selected by the density switch controls. While this approach has been used successfully in several photographic printers, it is intended primarily for a photographic printer which does not have computer controls for calculating exposure times. In addition, this system requires neutral density filters on the LATD sensors so that the outputs of the LATD sensors are matched.
There is continuing need for photosensor circuits in photographic printers which require no neutral density filters, which provide high accuracy in sensor reading, and which provide flexibility in the operation of the sensors under computer control.